Apparatus and method for determining the flow characteristic of a volumetric flowmeter

ABSTRACT

A mechanical displacement flowmeter calibrator has a first fluid line external of the measuring cylinder of the calibrator connected between the inlet and outlet thereof. A flowmeter that produces flow-representative pulses is connected in the fluid line. A rod is connected to a measuring piston adapted to travel through the measuring cylinder as a fluid barrier. The rod drives the measuring piston through the measuring cylinder at a predetermined, constant speed and thereby determines the flow rate of the calibration. The displacement of the measuring piston is sensed as it travels through the measuring cylinder during a test run, while the pulses produced by the flowmeter are counted during the time interval in which the piston displaces a given volume. The flowmeter is preferably connected in the fluid line at the pressure null point. First and second annular edge seals around the periphery of the measuring piston form an annular cavity into which pressurized fluid, preferably a lubricant, is injected. Before a test run, fluid flow in a second fluid line, external of the measuring cylinder, is induced, thereby establishing a closed loop around the first and second fluid lines and the measuring cylinder. To initiate a test run, fluid flow through the second fluid line is blocked to operate the measuring piston. Simultaneously therewith, the measuring piston is driven through the measuring cylinder from end-to-end to execute a test run.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.843,001 filed Mar. 24, 1986 and now U.S. Pat. No. 4,674,317 which is adivision of application Ser. No. 757,272 filed on July 19, 1985 and nowU.S. Pat. No. 4,627,267.

BACKGROUND OF THE INVENTION

This invention relates to the measurement of fluid flow and, moreparticularly, to the determination of the flow characteristic of aflowmeter.

In order to obtain accurate readings from a flowmeter, it must becalibrated from time to time by determining its characteristic, i.e.,the constant of proportionality between the flow rate of the fluidflowing through the flowmeter and the response given by the flowmeter,sometimes called the K-factor of the flowmeter. In the case of a turbinetype flowmeter that develops electrical oscillations proportional innumber to the volume of flow through the flowmeter, this characteristicis expressed in terms of the number of pulses generated by the flowmeterper unit volume of fluid passing there through. The flowmetercharacteristic is a function of the type of fluid, as well as the fluidtemperature, pressure, and flow rate, and varies as the parts of theflowmeter wear in the course of use. Apparatus to determine thecharacteristic of a flowmeter while in an operating fluid system iscalled a prover. Apparatus to determine the characteristic of aflowmeter in a self-contained system, i.e., not in an operating fluidsystem, is called a calibrator.

My U.S. Pat. No. 4,152,922 discloses a small-volume prover that employsmechanical volume displacement techniques. The prover has a measuringpiston that travels through a measuring cylinder as a fluid barrier insynchronism with fluid passing through the operating fluid system thatincludes the flowmeter under test. A rod connects the measuring pistonto a fluidically actuated control piston in a control cylinder whichserves to hold the measuring piston at the upstream end of the measuringcylinder between test runs and return the measuring piston to theupstream end of the measuring cylinder after each test run. When themeasuring piston is released at the upstream end of the measuringcylinder to start a test run, the momentum of the fluid flowing throughthe system rapidly accelerates the measuring piston to the same speed asthe fluid flowing through the measuring cylinder, which isrepresentative of the flow rate passing through the flowmeter. TheK-factor is determined by counting the number of pulses produced by theflowmeter during the time interval of a given volumetric displacement ofthe measuring piston.

Although the state-of-the-art of low-volume mechanical displacementprovers has rapidly advanced in recent years, the development ofsmall-volume calibrators has not kept pace. The large fluid volumerequirements often make it impractical to duplicate the actualconditions, i.e., pressure, temperature, and fluid type, in theoperating system of the flowmeter, so the K-factor must be derivedinferentially. In small-volume operation, drag represented by thefriction of the measuring piston and leakage across the measuring pistonadversely affect the accuracy and repeatablilty of the measurements.When the fluid is gas, these adverse effects are exacerbated by thecompressibility of gas. Compressibility also makes it difficult tocontrol the pressure of a gas passing through the flowmeter under test.

SUMMARY OF THE INVENTION

According to the invention a positive displacement flowmeter calibratorhaving first and second fluid lines external of the measuring cylinderof the calibrator is connected between the inlet and outlet thereof. Theflowmeter is connected in the first fluid line. A valve is connected inthe second line and a flow inducer, i.e., pump, is connected in one ofthe lines. Before a test run, fluid flow is induced by the pump, toestablish a closed loop around the first and second fluid lines. Toinitiate a test run, fluid flow through one of the fluid lines isblocked so the only path for fluid flow in the closed loop is throughthe measuring cylinder. Since the test fluid is already flowing throughthe flowmeter at the beginning of a test run, greater precision can beachieved in the calibrating process with a smaller volume of test fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of specific embodiments of the best mode contemplated ofcarrying out the invention are illustrated in drawings, in which:

FIG. 1 is a schematic diagram of one embodiment of a calibratorincorporating principles of the invention;

FIG. 2 is an enlargement of a portion of the embodiment of FIG. 1illustrating the double annular seal on the peripheral edge of themeasuring piston;

FIG. 3 is a schematic diagram of another embodiment of a calibratorincorporating principles of the invention;

FIG. 4 is a schematic diagram of still another embodiment of acalibrator incorporating principles of the invention;

FIG. 5 is a block diagram of apparatus for driving the measuring pistonduring a test run;

FIG. 6 is a schematic diagram of an alternative embodiment to FIG. 4.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

In FIG. 1, a measuring piston 10 mounted on a rod 12 is adapted totravel through a measuring cylinder 14 from end-to-end. A double annularseal 16 described in detail below in connection with FIG. 2, extendsaround the peripheral edge of measuring piston 10 to form an annularcavity. A seal housing 18 is mounted on the upstream end of measuringcylinder 14 and a seal housing 20 is mounted on the downstream end ofmeasuring cylinder 14. Double annular seals 22 and 24 are installed inhousings 18 and 20, respectively, around rod 12. Measuring piston 10travels through measuring cylinder 14 as a fluid barrier.

A control piston 26 is connected to the upstream end of rod 12. Controlpiston 26 travels through a control cylinder 28 mounted on the end ofseal housing 18 as measuring piston 10 travels through measuringcylinder 14. Control piston 26 has a peripheral edge seal 30 shownschematically.

At the downstream end of measuring cylinder 14, a linear optical encoderserves as a measuring piston displacement sensor for measuring piston10. The optical encoder comprises a stationary transparent elongatedruler 32 with opaque graduations mounted on the end of seal housing 20parallel to rod 12 and an optical transducer 34 mounted on the upstreamend of rod 12. Optical transducer 34, (which is a conventionaloff-the-shelf piece of equipment), comprises a light source 36 andlight-detecting diodes 38 and 40. By way of example, optical transducer34 could comprise Linear Optical Encoder DRC 1200 sold by DynamicsResearch of Wilmington, Mass.

Measuring cylinder 14 has an inlet at one end and an outlet at the otherend connected externally by a fluid line 42. Preferably, line 42approximately follows the shortest path between the inlet and outlet ofmeasuring cylinder 14, as shown, to minimize space requirements. Aflowmeter 44 to be calibrator, such as a turbine meter, is connected influid line 42. Fluid line 42 has fittings 46 and 48, which permits theinstallation of different flowmeters for calibration purposes. In anycase, to insure stable flow conditions in the flowmeter under test, thedistance between fittings 46 and 48 is about thirty times the diameterof the section of the fluid line therebetween, which depends upon theflow rating of the flowmeter under test.

An accumulator has a piston 50 that travels through a cylinder 52 as afluid barrier. Piston 50 has a schematically represented peripheral edgeseal 54. A compression spring 56 is disposed between an inlet end ofcylinder 52 and piston 50 to urge piston 50 toward an outlet end ofcylinder 52. Measuring cylinder 14 is connected to the inlet end ofcylinder 52 by a fluid line 58. The outlet end of cylinder 52 is alsoconnected by a fluid line 60, an axial bore 62 through the downstreamend of rod 12, and a radial bore 64 through measuring pistion 10 to theannular cavity around the periphery of measuring piston 10. The outletend of cylinder 52 is also connected to the annular cavities formed inseal housings 18 and 20, respectively, by fluid lines 66 and 68,respectively. The space between piston and the outlet end of cylinder 52is filled with a liquid lubricant, such as light lubricating oil. Aspring 56 pressurizes the liquid lubricant, which is distributed to theannular cavities formed by seals 16, 22, and 24. The liquid lubricant isat a higher pressure than the fluid in measuring cylinder 14. As thepressure of the fluid in measuring cylinder 14 changes, the force onpiston 50, and thus the pressure of the liquid lubricant changesaccordingly to maintain an approximately constant pressure differentialbetween the liquid lubricant and the fluid in measuring cylinder 14. Theliquid lubricant serves to reduce the frictional drag of measuringpiston 10 and rod 12 as piston 10 travels through measuring cylinder 14and the liquid lubricant pressure prevents liquid lubricant preventsleakage of the fluid in measuring cylinder 14 across measuring piston 10and to the exterior of measuring cylinder 14. A position sensor 70 whichcould be mechanical, optical, or magnetic in nature, indicates theposition of piston 50. A failure of seal 16, 22, or 24 occurs whenposition sensor 70 indicates a steady movement of piston 50 toward theoutlet end of cylinder 52.

A plenum chamber 72 having a volume much larger, i.e., 200 times ormore, than control cylinder 28, is charged with a pressurized gas.Plenum chamber 72 is connected to the top of a hydraulic accumulator 74.The bottom of accumulator 74 is connected by a throttling valve 76 tothe upstream end of control cylinder 28 to supply hydraulic fluid to theupstream face of control piston 26. A normally closed solenoid valve 78is connected in parallel with throttling valve 76 to provide a bypassthereto. The setting of throttling valve 76, which applies force to theupstream face of piston 26, determines the speed of measuring piston 10,after equilibrium has been achieved. The speed of measuring piston 10,in turn, determines the volumetric flow rate of the test fluid passingthrough flowmeter 44. Throttling valve 76 may be manually set or may becontrolled automatically by a servo loop in response to the linearencoder. The described apparatus permits calibration of the flowmeterunder test at a series of precisely determined volumetric flow rates. Athree-port, two-position solenoid actuated valve 80 controls pistonreturn after a test run. One port is connected by a fluid line 82 to thedownstream end of control cylinder 28. Another port is connected by afluid line 84 to a reservoir 86 for hydraulic fluid. Reservoir 86 isconnected by a filter 88 to the inlet of a constant pressure variabledisplacement pump 90, which is driven by an electric motor 92. Theoutlet of pump 90 is connected by a fluid line 94 to the remaining portof valve 80. Pump 90 produces a pressure greater than the pressure inplenum 72. Measuring cylinder 14 and fluid line 42 are charged with thetest fluid, e.g. gas to a predetermined pressure encompassed by thepressure range encountered in the operating fluid system in which theflowmeter being calibrated is to be used. Under some conditions, it isalso preferabe to use the same type of fluid for calibration as theflowmeter encounters in service in the operating fluid system and tomaintain the same temperature during calibration as the flowmeterencounters in such service. The features of the invention can be used toparticular advantage in calibrating a flowmeter with a gas as the testfluid, although the invention can also be used to advantage with aliquid as the test fluid, particularly if temperature uniformity is animportant factor.

In operation, a test run begins with measuring piston 10 at the upstreamend of measuring cylinder 14 and control piston 26 at the upstream endof control cylinder 28 (extreme right position as viewed in FIG. 1).Valve 78 is closed and valve 80 is in the position that supplieshydraulic fluid at constant pressure to the downstream face of controlpiston 26. Before initiating a test run, throttle valve 76 is set to thedesired flow rate for calibration. To initiate a test run, the positionof valve 80 is changed, thereby depressurizing the downstream face ofcontrol piston 26. As a result, the pressure exerted on the upstreamface of control piston 26 by the hydraulic fluid from accumulator 74drives measuring piston 10 from the upstream end of measuring cylinder14 to the downstream end thereof. As this occurs, the hydraulic fluiddownstream of control piston 26 flows through valve 80 into reservoir 86and the test fluid in measuring cylinder 14 flows through flowmeter 44in the direction indicated by arrows 96. As measuring piston 10displaces a given volume in measuring cylinder 14 determined by themeasuring piston displacement sensor, the number of pulses generated byflowmeter 44 is counted. The result in volume per pulse is the K-factorof flowmeter 44. Preferably, the double chronometry technique describedin my U.S. Pat. No. 3,403,544, is employed to make this count.

After calibration of flowmeter 44, it is disconnected from line 42 andinstalled in an operating fluid, e.g. gas, system. Preferably, the rangeof temperature and pressure of the operating system encompass the givenpressure and temperature during calibration and the fluid is the same asthe test fluid during calibration. As a result, the flowmeter iscalibrated under operating conditions, and no extrapolation ofcalibration data is required.

As shown, fluid line 42 is preferably symmetrically configured withrespect to the inlet and outlet ends of measuring cylinder 14. It hasbeen discovered that such an external fluid line has a pressure nullpoint along its length, essentially at the midpoint thereof, assumingsymmetry as described, at which no pressure changes occur due to thecompressibility of the test fluid in measuring cylinder 14. Flowmeter 44is located in fluid line 42 at the pressure null point so thatcalibration of flowmeter 44 occurs at a constant pressure despite thefact that the pressure upstream and downstream of measuring piston 10may vary due to compressibility. To fine tune fluid line 42 and thusmake the pressure null point precisely coincide with the turbine offlowmeter 44 after flowmeter 44 is installed in fluid line 42, anorifice 98 such as an orifice plate or an adjustable valve could beconnected in fluid line 42 near flowmeter 44. Such an orificecompensates for deviations in symmetry that may occur due to tolerancesin the dimensions of fluid line 42. This feature is also applicable to aball calibrator.

In a typical embodiment of the invention, the length of measuringcylinder 14 from end-to-end is sufficient for pistons 10 and 26 to havea 48 inch stroke and the inside diameter of measuring cylinder 14 is 12inches. The typical range of flow rates for such a calibrator is 0.03 to60 actual cubic feet of test fluid per minute.

In FIG. 2, the peripheral edge of measuring piston 10 has a groove 100opening toward the upstream end of measuring cylinder 14 and a groove102 opening toward the downstream end of cylinder 14. Annular retainingplates 104 and 106 having an outer diameter slightly smaller than theinner diameter of measuring cylinder 14 are secured to the upstream anddownstream faces of measuring piston 10, respectively. Annular supportrings 108 and 110 protrude slightly from the outer periphery ofretaining plates 104 and 106, respectively, to provide a bearing surfacethat supports piston 10 with a snug fit in measuring cylinder 14. Rings108 and 110 could be formed from Rulon tape wrapped around the outerperiphery of retaining plates 104 and 106. The periphery of measuringpiston 10 between grooves 100 and 102 is spaced from the inner surfaceof measuring cylinder 14 to provide fluid communication from bore 64 togrooves 100 and 102. U-shaped annular lip seals 112 and 114, which areretained in grooves 100 and 102, respectively, have legs that facetoward each other and bases that face away from each other. Sealexpanders 116 and 118, which are disposed within lip seals 112 and 114,respectively, spread their legs apart to bear against the inner surfaceof measuring cylinder 14 and the base of grooves 100 and 102,respectively. The liquid lubricant under pressure, represented bystipling in (FIG. 2, further) tends to spread the legs of lip seals 112and 114 apart so as to prevent leakage of test fluid across piston 10.By way of example, lip seals 112 and 114 could be teflon loaded withRulon fiber for strength and wear and seal expanders 116 and 118 couldbe noncorrosive stainless steel garter springs. A suitable doubleannular seal as illustrated in FIG. 2 is sold by Balseal of SantaMonica, CA, Model No. 307A-11.75-G.

Seals 22 and 24 are essentially the same as seal 16 with the exceptionthat the components of the seal are mounted on the stationary members,i.e., seal housings 18 and 20, rather than the movable member, i.e.,measuring piston 10. Thus, each seal housing has a pair of spaced apartannular grooves, as grooves 100 and 102, in which U-shaped lip seals, aslip seals 112 and 114, with inwardly facing legs urged apart by sealexpanders, as seal expanders 116 and 118, are disposed. Annular supportrings, as support rings 108 and 110, extend into the rod-receiving boreto provide bearing surfaces for the rod. The fluid line (66 or 68) leadsinto an annular cavity, as radial bore 64, formed by the grooves and arecess therebetween.

In the embodiment of FIG. 3, the components in common with theembodiments of FIG. 1 bear the same reference numerals. Instead ofdriving measuring piston fluidically as shown in FIG. 1, measuringpiston 10 is driven mechanically in this embodiment. Specifically, athreaded shaft 120 is rotatably driven by a motor 122. The threads ofshaft 120 engage threads at 126 on a translatable, nonrotatable carriage124, which is fixedly mounted on the downstream end of rod 12. Carriage124 rides on rails 128. The upstream end of rod 12 has a bore 130 thatreceives the portion of shaft 120 that extends beyond the threadedengagement at 126. A rotary encoder 132 is coupled to shaft 120, toserve as a measuring piston displacement sensor. The output of encoder132 is connected to a control console 134 by an electrical cable 133.The output of a pressure transducer 136, which senses the pressure inmeasuring cylinder 14, is connected to console 134 by an electricalcable 138. The output of a temperature transducer 140, which senses thetemperature of the fluid in measuring cylinder 14, is connected toconsole 134 by an electrical cable 142. The output of a ΔP transmitter144, which senses the fluid pressure difference between flowmeter 44 andmeasuring cylinder 14 is connected to console 134 by an electrical cable146. A ΔT transducer 148, which senses the fluid temperature differencebetween flowmeter 44 and measuring cylinder 14, is connected to console134 by an electrical cable 150. On the basis of the data transmitted toconsole 134 by cable 133, electronics in console 134 generates an errorrate control signal, which is applied to a motor control module 154 byan electrical cable 156. Motor control module 154 derives an actuatingvoltage for motor 122 responsive to the error rate control signal fromconsole 134. This control signal is applied to the actuating winding ofmotor 122 by an electrical cable 158. As a result, motor 122 drivesmeasuring piston 10 at a predetermined constant speed from the upstreamend to the downstream end of measuring cylinder 14 in the course of atest run to produce the data for deriving the K-factor of flowmeter 44,which is transmitted to console 134 by cables 133, 138, 142, 146, 150,and 152. By way of example motor 122 could be a three phase synchronousmotor and control module 154 could generate a three phase actuatingvoltage, the frequency of which varies as a function of the error ratecontrol signal; motor 122 could be Mode MC 345 sold by PMI Motors ofSyosset, N.Y. and control module 54 could be Model DMC 100 sold by GalilMotur Control, Mountain View, Calif. Although not shown, this embodimentalso employs accumulator piston 50, cylinder 52, spring 56, and fluidline 58 of FIG. 1.

A fluid line 160 is connected external of measuring cylinder 14 betweenits ends. A normally closed solenoid-actuated valve 162 is disposed inline 160. Line 160 preferably has a much larger diameter than line 42 sothat most of the fluid passes therethrough when measuring piston 10 isreturned to the upstream end of measuring cylinder 14 at the end of atest run. Console 134 is connected to the solenoid of valve 162 by anelectrical cable 164. When the electronics in console 134 detect thearrival of measuring piston 10 at the downstream end of measuringcylinder 14, valve 162 is opened and motor 122 drives measuring piston10 back to the upstream end of measuring cylinder 14 in preparation foranother test run. The output of encoder 132 is also connected to motorcontrol module to indicate when measuring piston 10 is at the upstreamor downstream end of measuring cylinder 14.

The mechanical measuring piston driving arrangement of FIG. 3 and thefluidic measuring piston driving arrangement of FIG. 1 areinterchangeable. Thus, if desired, the fluidic measuring piston drivingarrangement of FIG. 1 could be substituted for the mechanical measuringpiston driving arrangement of FIG. 3 and visa versa.

In the embodiment of FIG. 4, like components are identified with thesame references numerals as the embodiments of FIGS. 1 and 3. A fluidline 166 is connected external of measuring cylinder 14 from one endthereof to the other. Preferably line 166 has a substantially largercross section than line 42. A normally open solenoid-actuated valve 168is disposed in line 166 near the upstream end of measuring cylinder 14,a normally open solenoid-actuated valve 170 is disposed in line 166 nearthe downstream end of measuring cylinder 14, and a flow inducer pump 174is disposed in line 166 between valves 168 and 170. It is assumed thatcontrol piston 26 is driven and returned by the same fluidic drivingarrangement shown in FIG. 1, although the mechanical driving arrangementcould be used. Between test runs, valves 168 and 170 are open andmeasuring piston 10 is disposed at the upstream end of measuringcylinder 14. Pump 174 induces the test fluid to flow through line 166,measuring cylinder 14, and line 42 at the approximate flow rate at whichflowmeter 44 is to be calibrated. It is to be noted, however, that pump174 does not determine the flow rate of calibration--the driving speedof rod 12 does. To initiate a test run, valves 168 and 170 are closed toremove pump 174 from the test fluid circuit, and simultaneouslytherewith control piston 26 is actuated to drive measuring piston 10through measuring cylinder 14 at the prescribed speed to establish thedesired flow rate through flowmeter 44. (Actually, it may be desirablein practice to anticipate the closing off of line 166 by starting todrive measuring piston 10 a few milliseconds before closing valves 168and 170). In this embodiment, the test fluid is in motion at the timethat a test run is initiated and, therefore, flowmeter 44 can attainsteady-state operation more rapidly than the embodiment of FIG. 1. Thisembodiment is especially useful in calibrating a positive displacementmeter, which responds slowly to changes in flow rate, and is alsoapplicable to ball calibrators.

FIG. 6 illustrates an alternative embodiment to FIG. 4, like componentsbeing identified with the same reference numerals. In this embodiment,pump 174 and flowmeter 44, are both disposed in line 42 and a singlenormally open, solenoid actuated valve 168 is disposed in line 166. Inthis embodiment, pump 174 can be used in whole or in part to drive themeasuring piston. For more precise control over the constant speed ofthe measuring piston, the control system described below in connectionwith FIG. 5 could also be employed in this embodiment, as well as theembodiment of FIG. 4.

In the embodiments of FIGS. 4 and 6, lines 42 and 166 could either bothbe connected to each other through the upstream and downstream ends ofthe measuring cylinder 14 as illustrated in FIG. 4 or directly connectedto each other external of measuring cylinder 14 in which case, a singleconnection is made to the upstream and downstream ends of measuringcylinder 14 as shown in FIG. 1. In either case, these lines areconnected between the ends external of the measuring cylinder as thatterm is used in the claims.

The details of construction for the calibrators of FIGS. 1, 3, and 4 aredescribed in my following U.S. Pat. Nos., the disclosures of which areincorporated fully herein by reference: U.S. Pat. No. 3,403,544, whichissued on Oct. 1, 1968; U.S. Pat. No. 3,492,856, which issued on Feb. 3,1970; and U.S. Pat. No. 4,152,922, which issued on May 8, 1979. For avolume of about 3 cubic feet, a typical maximum piston speed in theembodiments of FIGS. 1 and 3 is about 2 feet per second and in theembodiment of FIG. 4 is about 4 to 5 feet.

FIG. 5 represents the system for controlling the speed of measuringpiston 14, either fluidically, as shown in FIG. 1, or mechanically, asshown in FIG. 2. The output of an encoder 176, which could be theoptical encoder of FIG. 1 or the rotary encoder of FIG. 2, is applied toa rate circuit 178 located in console 134. The output of rate circuit178 represents the actual speed of measuring piston 10. The output of areference signal source 180 in console 134 and the output of ratecircuit 178 are applied to a summing junction 182 in console 134. Theoutput of summing junction 182 is applied to a controlled element 184,which in the embodiment of FIG. 1 is the servo actuator for valve 76 andin the embodiment of FIG. 2 is motor control module 154. Thus, thecontrolled element drives measuring piston 10 at a constant speedprescribed by reference signal source 180, thereby establishing aconstant predetermined flow rate through the flowmeter under test.

The described embodiment of the invention is only considered to bepreferred and illustrative of the inventive concept; the scope of theinvention is not to be restricted to such embodiment. Various andnumerous other arrangements may be devised by one skilled in the artwithout departing from the spirit and scope of this invention. Althoughthe invention can be used to particular advantage to calibrate turbinemeters or other volumetric flowmeters, it can also be used to calibrateother types of flowmeters.

What is claimed is:
 1. A flowmeter calibrator comprising:a fluiddisplacement measuring cylinder; a fluid displacement measuring memberadapted to travel through the measuring cylinder from end to end insynchronism with fluid flow; means for sensing the displacement of themeasuring member as the measuring member travels through the measuringcylinder; a first fluid line external of the measuring cylinderconnected between the ends of the measuring cylinder; a flowmeterconnected in the first fluid line; a second fluid line external of themeasuring cylinder connected between the ends of the measuring cylinder;means in one of the fluid lines for inducing fluid flow to drive fluidthrough the flowmeter; and control means for alternately blocking andunblocking fluid flow through the second fluid line so the only path forfluid flow is through the measuring cylinder.
 2. The flowmetercalibrator of claim 1, in which the second fluid line has asubstantially larger cross-sectional area than the first fluid line. 3.The flowmeter calibrator of claim 1, additionally comprising means fordriving the measuring member through the measuring cylinder from end toend while the control means blocks the fluid flow.
 4. The flowmetercalibrator of claim 3, in which the driving means drives the measuringpiston through the measuring cylinder at a predetermined constant speed.5. The flowmeter calibrator of claim 4, in which the flow inducing meansis in the second fluid line.
 6. The flowmeter calibrator of claim 4, inwhich the flow inducing means is in the first fluid line.
 7. Theflowmeter calibrator of claim 1, in which the flow inducing means is inthe second fluid line.
 8. The flowmeter calibrator of claim 1, in whichthe flow inducing means is in the first fluid line.
 9. A method forcalibrating a flowmeter in a first fluid line connected between theinlet and the outlet of a positive displacement calibrator, the methodcomprising in the order recited, the steps of:connecting a second fluidline between the inlet and the outlet of the flowmeter to establishbetween the fluid lines a closed loop independent of the calibrator;inducing fluid flow in one of the lines to drive fluid through theflowmeter; and preventing fluid flow through the second line to operatethe calibrator without interrupting fluid flow through the flowmeter.10. The method of claim 9, in which the inducing step induces fluid flowthrough the first line.
 11. The method of claim 9, in which the inducingstep induces fluid flow through the second line.
 12. The method of claim9, in which the inducing step induces fluid flow approximately at agiven flow rate, the method additionally comprising the steps ofcalibrating the flowmeter at the given flow rate and thereafterinstalling the flowmeter in an operating fluid system through whichfluid flows at a range of flow rates encompassing the given flow rate.13. A positive displacement calibrator for calibrating a flowmetercomprising: first and second fluid lines; means for connecting the firstfluid line external of the calibrator between its ends; means forconnecting a second fluid line external of the calibrator between itsends; a flowmeter disposed in the second line; means for inducing fluidflow in one of the lines to drive fluid through the flowmeter; and meansfor preventing fluid flow through the second line to operate thecalibrator without interrupting fluid flow through the flowmeter.